im assuming you synced your ipod to your current (empty) itunes librbary.
well heres a quick lesson
Itunes does NOT have a function to EXPORT songs from your ipod. however you can delete them from your ipod. When you plug in oyur ipod a message will come up "Do you want to sync this ipod to your current library and lose all previously loaded songs?" (its worded diffrently but thats the gist of it) If you click yes then congrats you got a blank ipod to load up form your library. If you click no then your ipod is locked in itunes. You then have to go into ipod options and enable manual update. This unlocks the ipod and you can add remove music at your leisure. Now back to that message "Do you want to sync this ipod to your current library and lose all previously loaded songs?" if you clikced YES here then your ipod will constatly mirror itself to your itunes librbary (it wont do this on manual mode) that process is called Syncing or Autosync.
Since your ipod is blank now you cant get the music that came with your ipod back. So essensialy we cant do anyhting but prevent you from making a similer mistake sometime down the line.
best of luck
Music - 1366 songs - 3.4days - 6.73gig
Videos - 1Movie - 10Clips - 2.3hrs - 879.3mb
Totals - 1381items - 3.6days - 7.74gig
This software will backup your ipod to your comp and even rebuild playlists.